Field of the Invention
The invention relates to gas turbine engine variable bleed valves and, more particularly, to such valves used to prevent surge and remove ice from duct between booster and core engine compressor.
Description of Related Art
It is well known in the gas turbine engine field to provide variable bleed valves (VBVs), typically, doors that open to provide a bleed path to bleed off compressed air between the booster and core engine compressor of gas turbine engines. The air is often bled from what is referred to as a gooseneck flowpath between the booster and core engine compressor. Aircraft fan jet gas turbine engines and marine and industrial derivatives of such engines have employed various forms of curved flowpaths and VBV bleed doors that are retracted into the flowpath casing so as to form an entrance to a bleed duct that bleeds booster or low pressure compressor discharge airflow to draw particles out of the flowpath in a manner such as that disclosed in U.S. Pat. No. 4,463,552 entitled “Combined Surge Bleed and Dust Removal System for a Fan-Jet Engine” by Monhardt et al.
Because the bleed flow abruptly curves away from the direction of the compressor flow, it is very difficult to hold larger particles in the bleed flow because of their momentum. This problem is common to aircraft, marine, and ground based gas turbine engines. Turbofan jet engines, such as the General Electric CF6 and GE90 series of engines, have in series relationship a fan, a booster, and a core engine compressor, whereby a portion of the air passing through the fan flows into the booster and then the core engine compressor. In order to match the inlet airflow of the core engine compressor to its flight operational requirements and to prevent booster stall, a booster variable bleed valve (VBV) is provided in the form of a booster bleed duct having an inlet between the booster and the core engine compressor and an outlet to the fan duct.
Opening and closing of the booster bleed duct is conventionally provided by a circumferentially disposed plurality of pivotal doors that retract into the engine structure or casing and are operated by a single unison ring powered by one or more fuel powered actuators. Bellcrank linkages operably connect the retracting pivotal bleed doors to the unison ring. An example of such a stall prevention system using a retracting pivotal door, as compared to a sliding door or valve in the Monhardt patent, is disclosed in U.S. Pat. No. 3,638,428 entitled “Bypass Valve Mechanism” by Shipley et al. and assigned to the same assignee as the present invention and incorporated herein by reference. The operation of the VBV is scheduled by the engine controller, either a mechanical or digital electronic type may be used. The problem associated with conventional bleed valve ducts and valve doors is that larger particles and amounts of particles such as ice are often not drawn into the bleed duct. It is desirable to have an engine that provides the ability to remove large amounts of ice from the compressor airflow for accels and at high speeds to prevent icing stalls and flameouts and as well. It is desirable to efficiently bleed air between the booster and the core engine compressor to prevent booster stalls. Thus, it is highly desirable to remove ice from the gooseneck flowpath during accels to high speed without removing core airflow or minimizing the amount of core airflow that is removed.
Modern aircraft employ fewer of the higher thrust, fuel efficient, very-high-bypass engines such as the twin engine Boeing 767 aircraft. Aircraft with fewer engines require more total take-off power and more power per engine in order to satisfy the requirement of being able to fly with one engine out. Therefore, the engines are set to lower power settings resulting in less engine airflow during descent when all engines are operational. This results in high water content for engine airflow since the amount of ice, hail, or water that gets into the engine is the same so long as the aircraft speed remains the same.
On the other hand, higher bypass ratio engines have smaller core flow and larger bullet-nose frontal area. This means more ice, hail, or water gets through the compressor into the combustor resulting in higher water content for the air. These two fundamental phenomena combine to cause substantial increase of water-to-air ratio in the combustor resulting in such aircraft engines being more susceptible to engine flame out problem in rain or hail storms. The higher bypass ratio engines, having the large frontal area, also result in increased ice accretion on the booster inlet and booster stages during idle operation within an icing environment. This results in increased ice shed during the acceleration, including sheds at or near maximum power operation. It also increases the risk of compressor stall due to the ice sheds and more particularly to high speed rotor ice sheds, which historically has been an issue on two-shaft large engines, and will continue to be an issue on future large engines.
Modern high bypass ratio engines incorporate higher pressure core compressors and lower pressure boosters and, thus, produce less pressure difference between the booster exit and the fan bypass duct. This increases the difficulty of bleeding sufficient amounts of air from downstream of the booster to the fan bypass duct for protecting boosters from stall. The booster stall margin is controlled by opening the VBV doors to dump some of the booster flow overboard so as to control the booster operating line to a point below its stall line.
Thus, it is highly desirable to have a variable bleed valve and system for high bypass ratio engines incorporate higher pressure core compressors and lower pressure boosters bleed sufficient amounts of air from downstream of the booster for protecting boosters from stall. It is also highly desirable to have a variable bleed valve and system for such high bypass ratio engines able to prevent ice sheds and more particularly high speed rotor ice sheds from causing compressor stall or flame quenching in the combustor.